Water Worlds in Known Planetary Systems

byPaul GilsteronSeptember 7, 2006

Planetary migration — as when a ‘hot Jupiter’ moves inward toward its parent star during system formation — may not be as disruptive as we once thought. In fact, according to a new study led by the University of Colorado at Boulder and Pennsylvania State, solar systems with hot Jupiters may well harbor Earth-like planets covered with deep oceans.

The research team’s paper, just published in Science, paints a positively benign scenario, one in which the gas giant’s migration actually becomes the trigger for the formation of water worlds that could well turn out to be habitable. Let’s look at this more closely, because it’s quite a shift from earlier studies, which assumed that a hot Jupiter’s migrations would eject protoplanetary materials from the system or else absorb them.

Working with computer simulations, the Colorado/Penn State researchers now think the hot Jupiters force rocky debris outward in the system, helping the formation of rocky planets. At the same time, and this is crucial, the dense surrounding gas slows the orbit of small, icy objects in the outer reaches of the protoplanetary disk, causing them to move inward and deliver water to the newly formed rocky worlds. To achieve these results, the study used simulated protoplanetary disks containing more than 1000 rocky, icy bodies and ran them through 200 million years of planetary evolution based on the best information we have on how planets form.

“Upcoming space missions such as NASA’s Kepler and Terrestrial Planet finder and ESA’s COROT and Darwin will discover and eventually characterize Earth-like planets around other stars,” write the authors in Science. “We predict that a significant fraction of systems with close-in giant planets will be found to have a Hot Earth or potentially habitable, water-rich planets on stable orbits in the Habitable Zone.”

The remarkable conclusion is that one out of every three known exoplanetary systems may shelter Earth-like planets in its habitable zone. Habitability means temperatures that involve orbiting well outside the hot Jupiter’s orbit, but the team notes that rocky planets inside that orbit — ‘hot Earths’ — occur in the simulations as well.

Centauri Dreams‘ take: These results show again how early in the planet-hunting process we are, and how many assumptions we will have to revise as we proceed. Hot Jupiters themselves were almost inconceivable until first detected; now they comprise about 40 percent of all known exoplanets. It made sense to see their movements as inimical to life-bearing planets and some models assume this, but if they are not, our models of planetary formation begin to point to Earth-like planets in a wider variety of settings than we had ever imagined.

If they do exist, these would be water worlds with an exclamation point. The team’s findings show that such planets could have up to 100 times the water present on Earth today, oceans miles and miles deep, with all that implies about creating sustainable biospheres. One difference, however, is that these planets would probably have a lower percentage of iron than Earth, which could have significant effects on how their atmospheres evolved. We need space-based telescopes and spectroscopic studies to learn more.

Colorado researcher Sean Raymond has this to say about life on one of these water worlds:

“I think there are definitely habitable planets out there. But any life on these planets could be very different from ours. There are a lot of evolutionary steps in between the formation of such planets in other systems and the presence of life forms looking back at us.”

Just how different is something we can hope to learn when our technology produces viable propulsion methods for interstellar probes.

But read the whole study, one of the most heartening to come across my desk since Greg Laughlin and Jeremy Wertheimer’s work on Proxima Centauri. It’s Raymond et al., “Exotic Earths: Forming Habitable Worlds with Giant Planet Migration,” in the September 8 issue of Science. One can only imagine Webster Cash’s interest in this paper. The Boulder-based Cash’s New Worlds Imager design, now being considered by NASA, is just what we need in space to actually get an image of some of these water worlds.

Comments on this entry are closed.

philwSeptember 9, 2006, 9:11

I find it a little depressing. These allegedly myriad Oceania worlds might harbor rich ocean life (assuming life doesn’t form most readily in mineral rich shoreline tidal pools) but without the ability to harnes fire, a technological civilization is exceedingly improbable.

To me, “Earthlike” means seas of H2O, land and a rich all pervasive biosphere.

And then there’s the issue of whether there’s going to be enough stuff dissolved in those oceans to make life feasible. If the ocean gets too deep, an ice layer forms at the base, which would cut off the ocean from supplies of minerals from the silicate planet below.

This seems overly optimistic. I think it would be more accurate to say the study shows there may be more Earth-sized planets than previously thought. Saying these planets are Earthlike, meaning with conditions like Earth today, is pretty speculative.

I wonder if planets like this could even accumulate oxygen in their atmospheres.

On Earth, net O2 production occurs because of the burial of organic matter in sediments. Photosynthesis alone is not enough, since the unburied biomass is reoxidized on a short time scale, consuming the O2 that was liberated when it was formed.

Wthout exposed continents being subject to erosion, there will be much less sedimentation, so the rate at which the redox gradient from the atmosphere to the crust is ‘pumped’ will be much lower. Granted, the rate at which O2 is consumed by reduced material exposed by erosion will also be lower, but the rate at which reduced gases are emitted by volcanoes should not be.

Oxygen formation by photodissociation of water would produce oxygen, though if the ocean’s in contact with the underlying rocks it’d probably end up oxidising the rocks rather than building up in the atmosphere.

On the other hand, if there’s an ice layer then you could get build up of oxygen produced in this way.

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last twelve years, this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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